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Determining the timing of driver influences on 1.8-3.5 MeV electron flux at geosynchronous orbit using ARMAX methodology
  • Laura E. Simms,
  • Mark J. Engebretson,
  • Geoffrey D. Reeves
Laura E. Simms
Augsburg University

Corresponding Author:simmsl@augsburg.edu

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Mark J. Engebretson
Department of Physics, Augsburg University
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Geoffrey D. Reeves
Space Science and Applications Group, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
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Although lagged correlations have suggested influences of solar wind velocity (V) and number density (N), IMF Bz, ULF wave power, and substorms (as measured by AE) on MeV electron flux at geosynchronous orbit over an impressive number of hours and days, a satellite’s diurnal cycle can inflate correlations, associations between drivers may produce spurious effects, and correlations between all previous time steps may create an appearance of additive influence over many hours. Autoregressive-moving average transfer function (ARMAX) multiple regressions incorporating previous hours simultaneously can eliminate cycles and assess the impact of parameters, at each hour, while others are controlled. ARMAX influences are an order of magnitude lower than correlations. Most influence occurs within a few hours, not the many hours suggested by correlation. Over all hours, V and N show an initial negative impact, with longer term positive influences over the 9 (V) or 27 (N) h. Bz is initially a positive influence, longer term (6 h) negative effect. ULF waves impact flux in the first (positive) and second (negative) hour before the flux measurement, with further negative influences in the 12- 24 h before. AE (representing electron injection by substorms) shows only a short term (1 h) positive influence. However, when only recovery and after-recovery storm periods are considered (using stepwise regression), there are positive influences of ULF waves and V, negative influences of N and Bz, while AE shows no influence.